Pharmaceutical engineering – Textnotes

Distillation
Syllabus: Basic Principles and methodology of simple distillation, flash distillation, fractional
distillation, distillation under reduced pressure, steam distillation & molecular distillation

• Distillation is defined as the separation of the components of a liquid mixture by a process involving
vaporization and subsequent condensation at another place.
• The distillation process involves two steps; (a) converting a liquid into vapour phase and (b)
transferring the vapour to another place and recovering the liquid by condensation. The feed liquid
is known as Distilland. The condensed liquid is known as distillate or condensate.
• If one component is volatile and others are non-volatile, it is possible to separate volatile
components from non-volatile components by distillation. In such cases, distillation is considered
as a separation or purification method.
• When heat is supplied to a mixture, a more volatile liquid evaporates readily than the less volatile
liquid. As a result, the condensed liquid consists of a high proportion of highly volatile liquid and
less amount of less-volatile liquid. Therefore, distillation is said to be partial separation method. The
extent of separation is governed by the properties of the components involved and the physical
arrangements used for distillation.
Applications
1. Separation of volatile oils: Volatile oils are separated from cloves, anise seeds and eucalyptus
leaves by the method of steam distillation.
2. Purification of organic solvents: Normally, simple distillation method is used for the purification of
liquids having single component as a major fraction. Simple distillation method is also used for
determining the boiling range of a liquid as per IP, 1996, as a method to decide the purity. Absolute
alcohol (100% ethanol) can be obtained by azeotropic distillation.
3. Manufacture of official preparations: Spirit of nitrous ether and aromatic spirit of ammonia are
prepared by simple distillation. Distilled water and water for injection are prepared as per the
specifications of pharmacopoeia by simple and compression distillation methods.
4. Refining of petroleum products: In the petroleum industry, the crude oil is refined into different
fractions using flash distillation. Each fraction is a multicomponent system. E.g. petroleum ether 60
and 80.
5. Recovery of solvents: Solvents are used for extraction of drugs from plant parts and synthetic
reaction mixtures. These solvents must be re- covered, in order to prevent environmental
contamination. The recovered solvent may be recycled for further use.

General equipment for distillation

• Generally the Distillation equipment consist of following parts
❖ Still
It is a vaporizing chamber and used to place the material to be distilled. The size of the still should be
such that only one-half to two-thirds full of liquid is filled. If the still is too large, superheating and some
times decomposition of liquid may occur. The still is heated by a suitable means (e.g. steam) for the

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vaporisation of the volatile constituents. Stills are made of stainless steel, copper or suitable material
to provide efficient heat transfer.
❖ Condenser
• Condenser helps in condensing the vapour. Condenser is a heat exchanger. It is kept cold by
circulating water through water jacket. The boiling point and volatility of a substance are the main
factors governing the choice of the condenser. The main points in the construction of a condenser
are as follows.
1. The condenser must be easy to clean.
2. The cooling surface must be large enough, because rate of condensation is proportional to the
area of the condensing surface.
3. The condensing surface must be reasonably a good conductor of heat, because rate of
condensation is proportional to the rate at which the surface is cooled. For this reason, metal
condenser is preferred over glass, if suitable.
4. The film of condensed liquid is a bad conductor and must be re- moved quickly in order to avoid
serious impairment of the efficiency of condenser. For this reason, condenser is always placed
in an inclined position.
5. The warm water in contact with the condensing surface must be quickly carried away and its
place should be taken by fresh cold water.
6. The condenser is connected to the receiver through a suitable adapter.
❖ Receiver
• It is used to collect the distillate. It may be a simple flask or modified flasks such as Florentine
receivers. Sometimes, the receiver is immersed in an ice-bath or any other freezing mixture. This
minimizes loss of volatilization. Florentine receivers are used for the separation of oil and water.

Classification of distillation methods

1. Simple distillation
2. Flash distillation
3. Fractional distillation
4. Distillation under reduced pressure
5. Steam distillation
6. Molecular distillation

1. Simple distillation
• Simple distillation is a process of converting a single constituent from a liquid (or mixture) into its
vapour, transferring the vapour to another place and recovering the liquid by condensing the vapour,
usually by allowing it to come in contact with a cold surface.
• This process is known differential distillation, as distillation is based on the differences in volatilities
and vapour pressures of the components in the mixture. This method requires simple apparatus.
✓ Principle: Liquid boils when its vapour pressure is equal to atmospheric pressure. Simple
distillation is conducted at its boiling point. The higher the relative volatility of a liquid, the better is
the separation by simple distillation. Heat is supplied to the liquid so that it boils. The resulting
vapour is transferred to a different place and condensed. If the liquid of interest is volatile and
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remaining components are non-volatile, then simple. Distillation is a useful means of purification
and separation of liquids.
✓ Applications:
(1) Simple distillation is used for the preparation of distilled water and water for injection.
(2) Volatile and aromatic waters are prepared.
(3) Organic solvents are purified.
(4) A few official compounds are prepared by distillation. E.g. spirit
(5) Non-volatile solids are separated from volatile liquids.
✓ Construction: The construction of a simple distillation apparatus is shown in Figure 7-5. It consists
of a distillation flask with a side-arm sloping downwards. Condenser is fitted into the side arm by
means of a cork. The condenser is usually water condenser, i.e. jacketed for circulation of water.
The condenser is connected to a receiver flask using an adapter. On a laboratory scale, the whole
apparatus is made of glass.

✓ Working: The liquid to be distilled is filled into the flask to one-half to two-third of its volume. A
thermometer is inserted into the cork and fixed to the flask. The thermometer bulb must be just
below the level of the side arm. Water is circulated through the jacket of the condenser as shown in
Figure 7-5. The contents are heated gradually. The liquid begins to boil after some time. The vapour
begins to rise up and passes down the side arm into the condenser. The temperature rises rapidly
and reaches a constant value. The temperature of the distillate is noted down, which is equal to the
boiling point of the liquid. The vapour is condensed and collected into the receiver. The flame is
adjusted so that the distillate is collected at the rate of one to two drops per second. Distillation
should be continued until a small volume of liquid remains in the flask.
✓ Advantages: This process is economical as the amount of steam used in coils is reduced on
account of preheating of feed water by counter-current flow of the condensate. This also facilitates
the escape of dissolved gases without any additional effort.

2. Flash distillation
• Flash distillation is defined as a process in which the entire liquid mixture is suddenly vaporized
(flash) by passing the feed from a high pressure zone to a low pressure zone.

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• Flash distillation is also known as equilibrium distillation, i.e. separation is attempted when the
liquid and vapour phases are in equilibrium. This method is frequently carried out as a continuous
process and does not involve rectification.
✓ Principle: When a hot liquid mixture is allowed to enter from a high-pressure zone into a low-
pressure zone, the entire liquid mixture is suddenly vaporised. This process is known as flash
vaporisation. During this process the chamber gets cooled. The individual vapour phase molecules
of high boiling fraction get condensed, while low boiling fraction remains as vapour. This process
requires certain amount of time. Therefore, the liquid and vapour are kept in intimate contact until
equilibrium is achieved. The liquid fraction is collected separately. The vapour is separated from the
liquid and further allowed to condense.
✓ Uses: Flash distillation is used for separating components, which boil at widely different
temperatures. It is widely used in petroleum industry for refining crude oil.
✓ Advantages: Flash distillation is a continuous process. It is used for obtaining multi-component
systems of narrow boiling range, especially in oil refinery. Examples are petroleum ether 60, 80, etc.
✓ Disadvantages: Flash distillation is not effective in separating components of comparable volatility.
It is not suitable for two component systems. It is not an efficient distillation when nearly pure
components are required, because the condensed vapour and residual liquid are far from pure.
✓ Construction: The construction of a flash distillation apparatus is shown in Figure 7-8. It consists
of a pump, which is connected to a feed reservoir. Pump helps in pumping the feed into the heating
chamber which contains a suitable heating mechanism. The other end of the pipe is directly
introduced into the vapour-liquid separator through a reducing valve. The vapour outlet is provided
at the top of the separator and liquid outlet is provided at the bottom.

✓ Working: The feed is pumped through a heater at a certain pressure. The liquid gets heated, which
enters the vapour-liquid separator through a pressure-reducing valve. Due to the drop in pressure,
the hot liquid flashes, which further enhances the vaporisation process. The sudden vaporisation
induces cooling. The individual vapour phase molecules of high boiling point fraction get
condensed, while low boiling point fraction remains as vapour. The mixture is allowed for a sufficient
time, so that vapour and liquid portions separate and achieve equilibrium. The vapour is separated
through a pipe from above and liquid is collected from the bottom of the separator. By continuously
feeding into the still, it is possible to obtain continuous flash distillation. The operating conditions

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can be adjusted in such a way that the amount of feed exactly equals the amount of material re-
moved. Therefore, vapour and liquid concentrations at any point remain constant in the unit.

3. Fractional distillation
• Fractional distillation is a process in which vaporisation of liquid mixture gives rise to a mixture of
constituents from which the desired one is separated in pure form.
• This method is also known as rectification, because a part of the vapour is condensed and returned
as a liquid. This method is used to separate miscible volatile liquids, whose boiling points are close,
by means of a Fractionating column.
• Fractional distillation is different from simple distillation. In simple distillation, vapour is directly
passed through the condenser. In fractional distillation the vapour must pass through a fractionating
column in which partial condensation of vapour is allowed to occur. In simple distillation,
condensate is collected directly into the receiver, while in fractional distillation, condensation takes
place in the fractionating column, so that a part of the condensing vapour returns to the still.
✓ Principle: From the operational point of view, fractional distillation is a mass transfer process
involving counter-current diffusion of the components at each equilibrium stage. When a liquid
mixture is distilled, the partial condensation of the vapour is allowed to occur in a fractionating
column. In the column, ascending vapour from the still is allowed to come in contact with the
condensing vapour returning to the still. This results in enrichment of the vapour with the more
volatile component. By condensing the vapour and reheating the liquid repeatedly, equilibrium
between liquid and vapour is set up at each stage, which ultimately results in the separation of a
more volatile component.
✓ Applications: Fractional distillation is used for the separation of miscible liquids such as acetone
and water, chloroform and benzene.
✓ Disadvantage: Fractional distillation cannot be used to separate miscible liquids, which form
azeotropic mixtures.

✓ Construction: The assembly of apparatus for fractional distillation on a laboratory scale is shown
in Figure 7-12. The fractionating column is inserted between the still and the condenser. A provision
is made for the supply of heat (usually a steam coil) at the bottom of the column. At the top of

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column, a condenser is provided. The column has a large area for providing sufficient flow
conditions. The broken lines across the column represent the contacting devices.
• Working: ------
❖ Fractionating Columns
• Generally, it is necessary to conduct distillation several times by appropriate means in order to
separate a mixture of miscible liquids. This can be avoided by employing fractionating column for a
reasonably complete separation. In fractional distillation, special type of still-heads are required so
that condensation and revaporisation are affected continuously. These are known as fractionating
columns.
• A fractionating column is essentially a long vertical tube in which the vapour passes upward and
gets partially condensed. The condensate flows down the column and is returned eventually to the
flask. The columns are constructed so as to offer the following advantages simultaneously.
(1) It offers a large cooling surface for the vapour to condense. (2) An obstruction to the ascending
vapour allows easy condensation.
The obstruction also retards the downward flow of liquid, which Is a high boiling component.
• Fractionating columns can be divided into two groups.
Packed columns: In this type, some form of packing is used in the column to affect the necessary
liquid/vapour contact. The packing may consist of single turn helices (spirals) of wire of glass, glass
rings, cylindrical glass beads, stainless steel rings etc. The height of packing is equivalent to one
theoretical plate. A few types of fractionating columns are shown in Figure 7-15.

✓ Construction: Packed column consists of a tower containing a packing that becomes wetted with
a film of liquid, which is brought into contact with the vapour in the intervening spaces.
The same type of fractionating columns can be obtained in various lengths.
(a) A long fractionating column is necessary when the boiling points of the constituents Are lying
fairly close together.
(b) A short fractionating column is necessary when the boiling points of the constituents differ
considerably.
✓ Applications: Packing must be uniform so as to obtain proper channels. If packing is irregular, mass
transfer becomes less effective. Packed columns are mainly used in laboratories.

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❖ Bubble cap column

✓ Construction: The column consists of a number of plates mounted one above the other (Figure 7-
17). The plates have a weir leading to a down- comer. Caps are present on each plate, which allow
the vapour to escape by bubbling through the liquid.
✓ Working: Ascending vapour from the still passes through the bubble-caps on plate A and the rising
vapour will be richer in the more volatile component. This vapour passes through the liquid on plate
B and partially condensed. The heat of condensation partially vaporizes the liquid. The process of
condensation and vaporisation will be repeated at plate C and so on all the way up the column. Each
bubble-cap plate has the same effect as a separate still.
✓ Advantages: The bubble cap plate is effective over a wide range of vapour-liquid proportions and
velocities. There is an excellent contact as the vapour bubbles through the liquid.
✓ Disadvantages: (1) A layer of liquid on each plate results in considerable hold-up of liquid over the
entire column.
(2) The need to force the vapour out of the caps through the, liquid leads to a large pressure drop
through the column.
(3) The column does not drain even after completion of distillation.
(4) The structure is complicated making construction and maintenance expensive.

4. Distillation under reduced pressure

• Distillation under reduced pressure may be defined as a distillation process in which the liquid is
distilled at a temperature lower than its boiling point by the application of vacuum. Vacuum pumps
and suction pumps are used to reduce the pressure on the liquid surface. Distillation under reduced
pressure is based on the principle of simple distillation with some modifications.

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✓ Principle: Liquid boils when vapour pressure is equal to the atmospheric pressure, i.e. pressure on
its surface. If the external pressure is reduced by applying vacuum, the boiling point of liquid
decreases. Therefore, the liquid boils at a lower temperature. This principle is illustrated using an
example of water. Water boils at 100 °C at an atmospheric pressure of 101.31 kPa (760 mmHg). At
40 °C, the vapour pressure of water is approximately 9.33 kPa (70 mmHg). Hence, the external
pressure is reduced to 9.33 kPa (70 mmHg) where water boils at 40 °C. The net result is an increase
in the rate of mass transfer into vapour.

✓ Applications: Distillation under reduced pressure is essential and finds a number of applications.
✓ Disadvantages In vacuum distillation, persistent foaming occurs. This may be overcome by adding
capryl alcohol to the liquid or by inserting a fine air capillary tube in the second neck of the Claisen
flask. The stream of air is drawn in and breaks the rising foam. The above method is not suitable for
the preparation of semisolid or solid extracts.
✓ Construction: It consists of a double-neck distillation flask known as Claisen flask (Figure 7-18).
Thick walled glass apparatus with interchangeable standard glass joints are used for vacuum
distillation. In one of the necks of the Claisen flask, a thermometer is fitted. The second neck
prevents splashing of the violently agitated liquid. Bumping occurs readily during vacuum
distillation. Placing a fine capillary tube in the second neck of the flask can prevent bumping. The
capillary tube is dipped in the boiling liquid, so that a stream of air bubbles is drawn out. Water bath
or oil bath is used for heating. The Claisen flask is connected to a receiver through a condenser.
Vacuum pump is attached through an adapter to the receiver. A small vacuum gauge (manometer)
should be inserted between the pump and the receiver.
✓ Working: The liquid to be distilled is filled one-half to two-third volume of the flask. Small pieces of
porcelain are added to the liquid for facilitating distillation and prevent bumping. The capillary tube
and thermometer are kept in place in the flask (Figure 7-18). The required vacuum is applied. The
contents are heated gradually. The temperature rises and liquid gets vaporised rapidly due to
vacuum. The vapour passes through the condenser. The condensate is collected in the receiver. The
temperature is noted down, which would be less than the boiling point of the liquid. When a large
volume of a liquid is to be distilled under reduced pressure, it is more convenient to distil
comparatively small volumes at a time.

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5. Steam distillation
• Steam distillation is a method of distillation carried with the aid of steam and is used for the
separation of high-boiling substances from non-volatile impurities. High-boiling liquids cannot be
purified by simple distillation, since the constituents in the mixture tend to decompose at higher
temperatures. In such cases, steam distillation is employed. Steam distillation is used for the
separation of immiscible liquids. For substances, which are insoluble in water and not decomposed
by heat, steam distillation provides an alternative to distillation under reduced pressure. Steam
distillation is the most common example of differential distillation.

✓ Principle: A mixture of immiscible liquids begins to boil when the sum of their vapour pressures is
equal to the atmospheric pressure. In case of a mixture of water and turpentine, mixture boils below
the boiling point of pure water, though the turpentine boils at a much higher temperature than that
of water. For example, the boiling point of turpentine is about 160 °C. But when it is mixed with water
and heated, the mixture boils at about 95.6 °C. Al this temperature, the vapour pressure of water is
86.245 kPa (647 mmHg) and that of turpentine is 15.06 kPa (113 mmHg). The sum of the vapour
pressures is 101.31 kPa (760 mmHg) which is normal atmospheric pressure. Thus, high boiling
substances may be distilled at a temperature much below its boiling point, when water (steam) is
used. For volatile substances, which are miscible with water, steam distillation involves the same
principle as fractional distillation.
✓ Applications: (1) Steam distillation is used for the separation of immiscible liquids. E.g. toluene and
water. (2) This method is used for extracting most of the volatile oils such as Clove, anise and
eucalyptus. (3) It is useful in purification of liquid with high boiling point, e.g. Essential oil of almond.
(4)Camphor is distilled by this method. (5) Aromatic waters are prepared by this method.
✓ Advantages: Volatile oils can be separated at a lower temperature in steam distillation, without any
decomposition and loss of aroma. If a substance has low volatility, it can be satisfactorily distilled,
provided its molecular weight is considerably higher than water.
✓ Disadvantages: Steam distillation is not suitable when immiscible liquid and water react with each
other.
✓ Construction: The construction of apparatus for steam distillation on laboratory scale is shown in
Figure 7-20. It consists of a metallic ‘steam can’ fitted with a cork having two holes. Through one of
the holes, a long tube is passed so as to reach almost the bottom of the steam generator. This tube
acts as a safety tube, so that in case the pressure inside the steam generator becomes too much,
water will be forced out of it and the pressure will be relieved. Moreover, when steam starts coming
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out from the safety tube, it indicates that the steam can is almost empty. Through another hole, a
bent tube is passed. The other end of the bent tube is connected to the flask containing non-
aqueous liquid (for example, crude containing volatile oil) through a rubber bung. This tube should
reach almost the bottom of the flask. Through the other hole of the rubber bung, a delivery tube is
inserted which connects the flask and the condenser. The condenser is connected to a receiver
flask using an adaptor. Provisions are made to heat the steam can and flask.
✓ Procedure: The non-aqueous liquid is placed in the flask. A small quantity of water is added to it.
Steam can is filled with water. The steam generator and the flask are heated simultaneously, so that
a uniform flow of steam passes through the boiling mixture. The mixture gets heated. The steam
carries the volatile oil and passes into the condenser, which is cooled by cold water. The condensed
immiscible liquid is collected into the receiver. Distillation is continued until all the non-aqueous
liquid has been distilled. In the receiver, water and organic liquid form two separate layers, which
can be easily separated using a separating flask. For volatile substances, which are miscible with
water, distillation with steam would involve the same principle of fractional distillation.

6. Molecular distillation
• Molecular distillation is defined as a distillation process in which each molecule in the vapour phase
travels mean free path and gets condensed individually without intermolecular collisions on
application of vacuum. Molecular distillation is based on the principle of the simple distillation with
some modifications. This is also called evaporative distillation or short path distillation.
✓ Principle: The substances to be distilled in molecular distillation have very low vapour pressures.
Examples are viscous liquids, oils, greases, waxy materials and high molecular weight substances.
These boil at very high temperatures. In order to decrease the boiling point of the liquids, high
vacuum must be applied. The vapour pressure above the liquid is much lower than that of the
saturated vapour in equilibrium. At very low pressure, the distance between the evaporating surface
and the condenser is approximately equal to the mean free path of the vapour molecules. Molecules
leaving the surface of the liquid are more likely hit the condenser surface than to collide with other
molecules. Little or no re-condensation takes place at the surface of the liquid.
✓ Applications: Molecular distillation is used for the purification and separation of chemicals of low
vapour pressure.
(1) Purification of chemicals such as tricresyl phosphate, dibutyl phthalate and dimethyl phthalate.
(2) More frequently used in the refining of fixed oils.
(3) Vitamin A is separated from fish liver oil. Vitamin E is concentrated by this method from fish liver
oils and other vegetable oils.
(4) Free fatty acids are distilled at 10°C Steroids can be obtained between 100°C and 200°C while
triglycerides can be obtained from 200°C onwards. Proteins and gums will remain as non-
volatile residues. Thus, the above mixture can be separated by molecular distillation.
• Molecular distillation have following types:

Falling Film Molecular Still or Wiped Film Molecular Still

✓ Principle: In this method, vaporisation occurs from a film of liquid flowing down a heated surface
under high vacuum. The vapour (molecules) travels a short distance and strikes the condenser
nearby. Each molecule is condensed individually. The distillate is subsequently collected.

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✓ Construction: The construction of a wiped film molecular still is shown in Figure 7-22. The vessel
has a diameter of one metre. The walls of the vessel are provided with suitable means of heating
(jacket). Wipers are provided adjacent to the vessel wall. Wipers are connected to a rotating head
through a rotor. The condensers are arranged very close to the wall (evaporating surface) as shown
in Figure 7-22. Vacuum pump is connected to a large diameter pipe at the centre of the vessel.
Provisions are made for collecting the distillate and the undistilled liquid residue at the bottom.
✓ Working: The vessel is heated by suitable means. Vacuum is applied at the centre of the vessel and
wipers are allowed to rotate. The feed is entered through the inlet of the vessel. As the liquid flows
down the walls, it is spread to form a film by PTFE (polytetrafluoroethylene) wipers, which are moving
at a rate of 3 m/s. The velocity of the film is 1.5 m/s. Since the surface is already heated, the liquid
film evaporates directly. The vapour (molecules) travels its mean free path and strikes the
condenser. The condensate is collected into a vessel. The residue (undistilled or mean free path not
travelled) is collected from the bottom of the vessel and re-circulated through the feed port for
further distillation. Capacity is about 1000 L/h.

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Centrifugal Molecular Still
✓ Principle: In this method, liquid feed is introduced into a vessel. Which is rotated at very high speed
(centrifugal action). On account of heating, vaporisation occurs from a film of liquid on the sides of
the vessel. The vapour (molecules) travels a short distance and gets condensed on the adjacent
condenser. Each molecule is condensed individually. The distil- late is subsequently collected.

✓ Construction: The construction of a centrifugal molecular still is shown in Figure 7-23. It consists
of a bucket-shaped vessel having a diameter of about 1 to 1.5 m. It is rotated at high speed using a
motor. Radiant heaters are provided externally to beat the fluid in the bucket. Condensers are
arranged very close to the evaporating surface. Vacuum pump is connected to the entire vessel at
the top. Provisions are made for introducing the feed into the centre of the bucket, for receiving the
product and residue for re-circulation.
✓ Working: Vacuum is applied at the centre of the vessel. The bucket shaped vessel is allowed to
rotate at high speed. The feed is introduced from the centre of the vessel. Due to centrifugal action
of the rotating bucket, liquid moves outward over the surface of the vessel and forms a film. Since,
the radiant heaters heat the surface, the liquid evaporates directly from the film. The vapour
(molecules) travels its mean free path and strikes the condenser. The condensate is collected into
another vessel. The residue is collected from the bottom of the vessel and is re-circulated through
the feed port for further distillation.
✓ Disadvantages Construction and operation are more complicated compared to falling film
molecular still.

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